This invention relates to multilamp photoflash units having circuit means for igniting the flashlamps, and more particularly, to high-voltage photoflash array with improved means for providing electrostatic protection.
Numerous multilamp photoflash arrangements with various types of sequencing circuits have been described in the prior art; particularly, in the past few years. A currently marketed photoflash unit (described in U.S. Pat. Nos. 3,894,226; 3,912,442; 3,935,442; 3,937,946; 3,941,992; 3,052,320 and 4,017,728 and referred to as a flip flash) employs high-voltage type lamps adapted to be ignitied sequentially by successively applied high-voltage firing pulses from a source such as a camera-shutter-actuated piezoelectric element. The flip flash unit comprises an elongated planar array of eight high-voltage type flashlamps each having a pair of lead-in wires connected to a printed circuit board by means of eyelets thereon. The circuit board is provided with switching circuitry for causing sequential flashing of the lamps, and an array of respectively associated reflectors are positioned between the lamps and the circuit board. The reflectors for the lamps can be made as a single reflector member shaped to provide multiple individual reflectors for the lamps. The reflector member preferably is electrically conductive, such as being made of metal or metal-coated plastic, and is electrically connected to an electrically "ground" portion of the circuitry on the circuit board; thus, the reflector member functions as an electrical shield reducing the possibility of accidental flashing of the lamps by an electrostatic voltage charge on a person or object touching or near the unit. Such accidental flashing is particularly prone to occur in this instance as the primers used in the high-voltage type flash-lamps employed in such arrays are designed to be highly sensitive toward high-voltage dishcarges. Electrical energies as low as a few microjoules are sufficient to promote ignition of such primers and flashing of the lamps. This high sensitivity is needed in order to provide lamps that will function reliably from the compact and inexpensive piezoelectric sources that are practical for incorporation into modern miniature cameras. Typically, the high voltage pulses provided by the camera are in the order of 500 to 4000 volts.
The use of a conductive reflector unit as an electrostatic shield for the flipflash array is described in the aforementioned U.S. Pat. No. 3,935,442 wherein the reflector is connected to a ground point of the circuit board by means of a conductive U-shaped spring clip which engages a web portion of the reflector unit and extends against a conductive area on the circuit board. This clip-engaged conductive area on the circuit board comprises an enlarged portion of a common circuit conductor which is connected to one lead-in wire of each of the flashlamps. An alternative method of connecting the reflector to a "ground" point of the circuit board is described in the aforementioned U.S. Pat. No. 3,941,992, wherein FIG. 2 thereof shows a wire 77 connected between the metallized reflector and a common run. Both of these reflector grounding methods share a basic deficiency in that initial electrical connection is not always established due to inevitable distortion of the plastic array components during manufacture and assembly. Loss of this mechanical (and electrical) contact renders the lamps of that array subject to unintentional electrostatic flashing.
Another problem found with these grounding methods is that the electrical contact is readily lost under humid conditions, principally because of galvanic corrosion of the thin, aluminized reflector coating at the point of contact. A further disadvantage of the U-clip approach is its compartively higher cost.
Another reflector grounding method, described in U.S. Pat. Nos. 4,093,979 and 4,104,706, places the conductive surface of the reflector as closely as possible to the common circuit conductor. In alternative embodiments, a tab integral with the conductive reflector resiliently engages against the common circuit conductor. U.S. Pat. No. 4,060,721 also describes a reflector arrangement with a resilient tab for grounding. All of these methods of grounding suffer from similar disadvantages to those mentioned above. Further, the approach of bringing the reflector close to the common conductor relies on what is essentially a high voltage spark gap rather than a reliable low voltage path to ground. The resilient tabs pose a problem because of the thinness of the aluminizing. Any flexing of such a thin conductor on the present polystyrene material would cause discontinuities, and the aluminizing deteriorates during humidity tests when it just touches the circuit material.
A copending application Ser. No. 866,262 filed Jan. 2, 1978 now U.S. Pat. No. 4,189,298 and assigned the same as the present invention, provides an improved reflector grounding arrangement in which a pliable, electrically conductive adhesive material bridges the gap between the reflector and common circuit conductor and provides a reliable low voltage connection therebetween. Although advantageous for use in the flip flash units described in the aforementioned patents, the conductive adhesive connection is not as suitable for use with photoflash units of the type described in the cross-referenced copending applications Ser. Nos. 840,497 Audesse et al, 840,498 Armstrong, 860,759 Brower, and 861,652 Sindlinger et al. These cross-referenced applications describe an improved multilamp photoflash unit which more efficiently utilizes a given housing volume and thereby reduces the cost of the unit per flashlamp contained therein. More specifically, a compact lamp arrangement is provided whereby additional lamps are contained in a given volume while maintaining light output performance requirements. In a particular embodiment described, ten lamps are provided in a housing having the same dimensions as the above-discussed eight-lamp flip flash units. The greater compactness is provided by arranging the planar array of lamps in two parallel columns with the tubular envelopes horizontally disposed and with the lamps of one column staggered with respect to the other such that the bases are interdigitated. A pair of reflector panels are aligned with the two columns of lamps and arranged overlie the lamp lead-in wires and bases.
As may well be appreciated, the above-described compact ten-lamp array imposes a significant challenge with respect to packaging design. Also, the circuit must be laid out in a very dense pattern on the lamp mounting surface of the associated circuit board. Another difference in the assembly is that the lamps are mounted for being disposed horizontally in the finished array, whereas in the earlier-mentioned eight-lamp array, the lamps are disposed vertically and respectively assembled in the multiple cavities of a single reflector unit. In the ten-lamp array, the two reflector panels comprise the two halves of the total reflector array and are joined together from each side onto the lamp-circuit board assembly. There is no lamp-fitting hole in each reflector panel since the lamps are positioned directly into the reflector cavities as the panels are positioned onto the circuit board.
With respect to grounding of the conductive reflector to the circuit board common conductor, the conductive surfaces that are connected by conductive adhesive in the eight-lamp arrays lie in parallel planes one above the other. The one-piece reflector is placed on the circuit board before the lamps are bent over. This arrangement, along with the large reflector openings, allows the conductive adhesive to be deposited on both the reflector and circuit board when assembled. In contrast, the two-piece reflector of the ten-lamp array is assembled after the lamps have been bent over. Because of the physical characteristics of the conductive adhesive, it is not feasible to try to deposit it on the reflector and then attempt to assemble the array.